Sunday, January 8, 2017

ASASSN-15lh. The Brightest Supernova Ever Seen.

Massive
stars are not known for going gentle into
that good night,
but the newly discovered supernova ASASSN-15lh is taking things to a whole new
physics-defying level.

According
to a new study published today in Science, ASASSN-15lh
shone with the light of 570 billion Suns during its peak last June (the
equivalent of over 20 times the output of the entire Milky Way galaxy) making
it at least twice as luminous as the next brightest supernova in the books.

Put simply,
this thing is the heavyweight champion of supernova—catastrophic explosion of
stars at the end of their lifetimes—and it challenges everything we know about
them.

But for
Dong and his colleagues, the find turned out to be as baffling as it was
thrilling. ASASSN-15lh belongs to a special category of phenomenon called super
luminous supernovae (SLSNe), or hyper novae, which are hundreds of
times more
energetic than regular supernovae, and much rarer. Only a few dozen examples have been detected.

Astronomers
still aren’t sure what powers these extraordinary bursts, especially on the
record-breaking levels observed in ASASSN-15lh. But one popular theory suggests
that they can be chalked up to a special class of neutron stars known as
magnetars.

In addition
to having one of the coolest names in astronomy, magnetars are mind-boggling
objects thought to be so dense that a spoonful of
their material
would weigh 200 billion pounds Their magnetic fields are roughly 1,000 trillion
timesmore
powerful than those generated by Earth, resulting in crustal star quakes that
create blasts of highly energetic X-ray and gamma ray light. These objects can
also spin very rapidly, rotating once every millisecond in some dizzying cases.

All of
these extreme properties add up to magnetars
being solid candidates as the engines of a hyper novae because they have so much potential
energy. The idea is that as magnetars spin down, or slow their rotational
period, energy is transferred into an envelope of magnetized wind and ejected
material surrounding them. The effect is like the Northern Lights on steroids—an
incredibly energetic thermal interaction that produces massive bursts of
optical energy.

But there’s
a big hole in this theory, because previous
research suggests
that a magnetar-induced hyper nova can only produce so much energy before it
starts to infringe on the laws of physics, and that number is easily exceeded
by ASASSN-15lh.

“Some of my
colleagues think that it is plausible to push the magnetar model parameters to
an extreme to interpret it—in that case, this would be a magnetar rotating at
the maximum allowed spin speed by physics, that has the strongest magnetic
field for any neutron stars people have observed, and also needs to radiate at
100 percent efficiency,” Dong explained.

That’s a
lot of perfect conditions to hit, which throws the magnetar hypothesis into
doubt, not just in the case of ASASSN-15lh, but potentially for all super
luminous supernova.

To that
point, another explanation put forward by Dong’s team is that the decay of
unusually large stockpiles of nickel into the isotope nickel-56 (Ni-56) is
driving the insane radiance of ASASSN-15lh.

This highly
luminous nickel decay is well-observed in normal supernovae, but has yet to be
understood in relation to explosions on the scale of hyper novae. Fortunately,
ASASSN-15lh provides an excellent opportunity to investigate this possibility.
Over the coming months, astronomers will attentively watch the supernova as it
expands, to see if the process exposes emission lines that suggest nickel decay
is the source of its brightness. They should know within the year.

At the
moment, ASASSN-15lh remains a perplexing mystery, with an outsized grandeur
that seems to strain against the hallowed physical rules of the universe. The proverbial scientific wisdom is that extraordinary claims
require extraordinary evidence, but events like this seem to turn that idea on
its head. What astronomers have uncovered is extraordinary evidence, and
extraordinary claims will be required to explain it.

“In the
history of science, great mysteries have inspired new insights into nature and
spurred new observational and experimental efforts to push the boundary of the
known,” Dong told me. “I hope the discovery of ASASSN-15lh will motivate people
to re-examine the thinking’s behind the supernova explosion theories and come
up with fresh ideas. ASASSN-15lh is a sample of only one, and I wish more
exciting discoveries like it will follow in the near future.”

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